Objectives

10/14/2013
Objectives
Eleven Pearls for the
Low Vision Evaluation
William L. Brown, O.D., Ph.D., F.A.A.O.
Disclosure Statement:
• Nothing to disclose
Please silence all mobile devices.
Unauthorized recording of this
session is prohibited.
#1 Be flexible with refraction
techniques
•
•
•
•
Just noticeable difference (JND)
Illumination levels
Refraction options
Use working distance with known add power
to estimate accuracy of distance Rx
At the completion of this course, the participant will be able
to:
• Understand the importance of discussing the functional
implications of the eye disease.
• Understand the differences between Galilean and
Keplerian telescopes regarding ease of aiming.
• Use the closed circuit television to determine an
appropriate power for a stand magnifier.
• Discuss methods for making the refraction more
effective.
• Discuss when bar magnifiers may have advantages over
spherical stand magnifiers.
Respect just noticeable
difference (JND)…
but don’t be limited by it
• Large interval sphere changes
– JND = (denominator of Snellen acuity with 20 in numerator)/100
– Example: VA 10/200  20/400
JND: 400/100 = 4.00 D
VA
JND Lens
Use +/- 2.00 D
intervals
– If patient can’t respond
that either change makes
20/100 1.00 ± 0.50
it worse, try larger interval, 20/200 2.00 ± 1.00
maybe +/- 3.00 D
20/400 4.00 ± 2.00
#1 Be flexible with refraction techniques
Respect just noticeable difference (JND)…
+/- 0.50
+/- 1.00
+/- 0.25
but don’t be limited by it
• Large interval JCC
VA
JCC
20/40 or better
± 0.25
20/50-20/100
± 0.50
Poorer than
20/100
± 1.00
(or greater)
#1 Be flexible with refraction techniques
#1 Be flexible with refraction techniques
Illumination levels
• Be sensitive to patient
preference for chart/room
illumination
– Higher levels are often helpful
for macular degeneration
– Dim levels for rod monochromats,
or others who like less light
• Avoid glare
– Well directed illumination
– Well shielded illumination
• Educate while testing!
#1 Be flexible with refraction techniques
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10/14/2013
Refraction options
Refraction options – Trial frame
• Large lens changes convenient
– Consider lens flippers
• Allows eccentric viewing
• Troublesome if trial frame
does not fit in focimeter
to determine power
• Allows habitual head
position
• Phoropter
– Stable
– Comfortable
– Large interval lens changes clumsy
• Unless trial lenses used
– Limited allowance for
• eccentric viewing
• head turn to null point for nystagmus
• Pantoscopic tilt (PT)
– Plus lens –  PT   + cyl X 180
– Minus lens –  PT   - cyl X 180
– Difficulty monitoring for eccentric
viewing
#1 Be flexible with refraction techniques
Refraction options – Trial lens clips
• Helpful for high
power corrections
with existing glasses
with vertex distance
already set.
• Stability of loose
cylinders potentially
troublesome
• Sometimes difficult to
fit in focimeter.
– 86 yo sc clearest reading distance = 25 cm
– 1/.25 m  4 D myopia
#2 Use log MAR (Minimum angle of resolution)
charts for refraction when
possible.
– Geometric
– Every 3 lines changes size
by factor of 2
Use working distance with known add
power to estimate accuracy of distance Rx
• Example: 80 year old pt wears +3.00 add, but
holds print at 20 cm for best near visual acuity.
• Effective add is 1/0.20 m =+5 D
• Likely that the distance power of the glasses is
over-plussed by approximately 2 D
• Also consider far point location to estimate
amount of myopia.
#1 Be flexible with refraction techniques
• Multiple letters
• Portable charts allow
several letters at the acuity
level
• Similar legibility
• Constant proportional
change in letter size
between lines
• Constant proportional
spacing between letters
#1 Be flexible with refraction techniques
#1 Be flexible with refraction techniques
#3 Be prepared to discuss the
physical and functional
characteristics of the eye
disease.
*3 lines
*0.3 log
units
*2X
• Don’t assume this has been done to patient’s
level of understanding.
• Patient angst and uncertainty is an important
issue.
#2 Use log MAR charts.
#3 Be prepared to discuss the physical and functional
characteristics of the eye
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10/14/2013
Describe the expected
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•
•
•
•
•
•
- Gives patient confidence that the suggestions
from the examination are founded on an
understanding of the patient’s situation.
#4 Use closed circuit television to
determine the equivalent
power needed for reading.
Blind spots
Color vision decline
Jumpy vision
Glare
Poor dark adaptation
Poor contrast
Interference of poorer eye on binocular vision
• When large amounts of
magnification are needed for
reading, but patient struggles
during testing with high add.
• Determine electronic
magnification (EM) needed for
fluent reading.
• Feq = add x EM
#3 Be prepared to discuss the physical and functional
characteristics of the eye
Example: Newsprint read fluently
50 cm from screen. Image on screen
is 12X larger than newsprint.
• Feq = add x EM
= (+2 D)x 12 = 24 D
• Find enlargement ratio (ER) needed for a stand magnifier
(SM), for example, if patient wears +3.00 D add
Feq = +24 D = add x ER
ER = (24 D) / (+3D) = 8
• Select SM with ER equal to or larger than the calculated ER.
See Berkeley “Low Vision Yellow Pages” (Bailey I et al)
http://optometry.berkeley.edu/general/technical-amiscellaneous-documents-and-materials
#4 Use CCTV to determine equivalent power needed for reading.
Stand magnifier used at optimal
and sub-optimal eye-SM distances
View at optimal distance
#4 Use CCTV to determine equivalent power needed for reading.
Example: Newsprint read fluently
on CCTV with 12X EM, sitting 50 cm from
screen.
Need ER for SM  8
• Select SM from your labeled magnifiers,
with ER  8. Some candidates:
Illuminated Stand
Magnifiers
Equiv
SM to image
Power (D) distance (cm)
Lens to eye
needed for
3.00 add (cm)
ER
Lens
size
(mm)
Peak 2023 15X
46
16
17
8.4
18
COIL 6329 14.7X
52
15
18
8.9
28
Eschenbach 1551
7X/28D
27
31
2
9.6
35
Schweizer 162 8X
26
28
5
8.3
27
#4 Use CCTV to determine equivalent power needed for reading.
#5 Aim for sufficient reading
reserve.
• Don’t stop at print size patient desires to
read.
• Target at least 0.3 log unit smaller on
high contrast acuity chart
– 3 lines lower on a logMAR chart
(Represents 50% the goal print size).
60 = eye to lens distance in
mm for image to be 400 mm
from eye for 2.5 add
• Eschenbach data: lens to
image = 340 mm
View at sub-optimal distance
#5 Aim for sufficient reading reserve.
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Minimum acuity reserves
Reading performance Minimum
w/ magnification
acuity
reserve
Required
threshold
size to
read 1M
Required
threshold
size to read
2M (large
print)
Spot
1.3:1
(1 line)
0.8 M
1.6 M
Fluent
2:1
(3 lines)
0.5 M
1.0 M
Maximum or
near maximum
3:1
(5 lines)
0.3 M
0.6 M
• Confirm adequate reading performance with
low contrast newspaper or print of patient
preference.
Lovie-Kitchin JE & Whittaker SG. Prescribing near magnification for low
vision patients. Clin Exp Optom. 82.6 Nov-Dec 1999, 216.
#5 Aim for sufficient reading reserve.
#6 Emphasize good lighting
#7 Consider recommending electronic
e-readers
• LED magnifiers
Device
– Highly important advancement in
usefulness of magnifiers
– Generally superior than incandescent
or halogen illumination
• Well directed light onto the page for
spectacle magnification
• LED flashlights
• Caution regarding reflections from
overhead lights off magnifier surface
#6 Emphasize good lighting.
Resources for e-readers
• See Calibre (http://calibre-ebook.com/) to change file formats,
customized fonts, font sizes, etc
• www.mobileread.com – helpful forum on e-readers
• Free Kindle reading apps for most devices
– Search “free Kindle reading apps”
• Web sites to check combinations of font/background colors
– http://www.pagetutor.com/colorpicker/index.html
– http://bcn.boulder.co.us/community/tools/colors/sccscgrn.htm
• Stands for tablets:
https://docs.google.com/document/d/1QLQAaT7jA9Mm1z9l
I13UqpcSfnKW3itmilAIAtT9_Vk/edit?pli=1
#7 Consider e-readers.
Kindle
Paperwhite 3G
IPad Mini
Screen Light
size
source
(“ diag)
6
External
&/or
self
front ill.
8
Back-ill.
Wt
(oz)
Recharge
time (hr)
# file
#
formats fonts
#
Color
font
sizes
7.8
4.5
7
6
6
N
10.9
2
9
6
16
Y
Kindle Fire 7, 9
Back-ill. 13.9 4
7
Y
HD
Nook HD 7
11.1 4
5
Y
Good listings of suggestions for ereaders
1)https://docs.google.com/document/d/1pb6EsVae52YMuURy68lNhEn8gy50ARaJRAnoZ1qmZ4/edit?pli=1
2)http://www.amazon.com/forum/kindle?_encoding=UTF8&cdForum=Fx1D7SY3BVSESG&c
dThread=Tx2I8V3ZDJJYB6J
#7 Consider e-readers.
#8 Discuss computer accessibility
options
• Changing font size on web site
– Examples:
Windows Explorer – Use Control (+) to magnify, Control (-) to minify
Mac – Use Command (+) to magnify, Command (-) to minify
• Changing font size in a program
– Check program Home menu for Change Font size or equivalent
– Zoom feature – some programs have zoom bar, often in the View
menu
• Screen magnifier options
• Accessibility Choices
–
–
–
–
For screen magnifier, screen resolution, color contrast settings
Windows XP: Accessibility Options (Control Panel)
Windows 7 , Vista: Ease of Access Center (Control Panel)
Mac OS X 10.4 & later: Universal Access (Settings menu)
See Lighthouse tips:
http://www.lighthouse.org/downloads/resources/computer.pdf
• Screen magnifying software
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10/14/2013
Hand magnifier – add power interaction
#9 Be aware of multifocal add/ hand
magnifier (HM) power interaction
• Maximum Feq when magnifier against add (zero
separation)
• Feq decreases by “add” worth of power for each unit
increase in separation (t) of add and HM
– Where one unit is a focal length of the magnifier.
HM at 1 focal length of HM from add
Feq = (FM + FA) – FA = FM
HM at add – Maximum Feq = FM + FA
• Example: +10 D hand magnifier (focal length = 10 cm)
with +2.00 add.
– With magnifier against add, t = 0, Feq = +12 D
– With magnifier held 40 cm from add, t = 4 focal lengths
Feq = +12 D – 4(2) = +4.00 D, much less than the magnifier
alone
– Pt is better to use no add with the magnifier, or a stronger
magnifier than otherwise expected may be needed.
HM at 4 focal lengths of HM from
add Feq = FM - 3FA
HM at 2 focal lengths of HM from
add : Feq = FM - FA
#10 Consider Galilean telescope (TS)
for seniors
Exit pupil locations
Keplerian and Galilean TS
F o’
Fe
• Proper placement of patient
entrance pupil in alignment
with TS is challenging for
many seniors.
• Exit pupil for Galilean TS is
inside TS, so patient’s
entrance pupil need not be
as accurately placed as with
Keplerian TS.
Objective
Exit pupil
Objective
• Plane of image seen through magnifier close to object
plane, so easy to focus back and forth between
magnified and unmagnified images.
• Potentially useful for patient with island of vision
– Spherical magnification pushes magnified image out of
island of vision.
– Bar magnifier maintains same visual span horizontally
while magnifying vertically.
#11 Recognize patients who may benefit from a bar magnifier.
F o’
Fe
Eyepiece
Galilean TS
#10 Consider Galilean TS for seniors.
•
51 yo CF, long history of Stargardt’s
•
CC: Trouble reading large print, using combination of reading
glasses & illuminated Ott Light Magnifier
 Would like stronger reading glasses to replace this combination –
too cumbersome and print difficult to read
• Magnification in one direction only.
• Magnification = index of refraction of magnifier
– Index for all magnifiers close to 1.5, so magnification is
approximately 1.5X
– Despite the physical size of the magnifier, magnifications
are virtually the same.
Eyepiece
Keplerian TS
#10 Consider Galilean TS for seniors.
#11 Recognize patients who may
benefit from using a bar magnifier.
Exit pupil
 Content to continue reading large print. Size = ?
•
2M
VA with current glasses
1) Bifocal
RT 5 ft/125
LT 5 ft/40
-VA best corrected 6 mo earlier with same glasses:
RT 20/400 (PH 20/200)
LT 5 ft/200E
Eccentric viewing both eyes
•
2) Near only glasses
LT (power = +2.50 add) 0.23/2.0 MNRD
•
Rfn: RT 1 D less minus
LT no change
(5 ft/80)
(5 ft/40)
#11 Recognize patients who may benefit from a bar magnifier.
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Effect of increasing magnification 1.5X overall vs 1.5X vertically on visual span
Cylindrical magnifiers
Original visual span 8 letters
Four score and seven years ago our fathers
brought forth on this continent a new nation,
• Curvature in one principal
meridian only
conceived in liberty, and dedicated to the …
Magnify 1.5X
overall – cuts
visual span to
5 letters
Magnify 1.5X
vertically only
– maintains 8
letter visual
span
– Magnification in curved
meridian only
• Characters enlarged
Four score and seven years ago our fathers
brought forth on this continent a new nation,
conceived in liberty, and dedicated to the …
#11 Recognize patients who may benefit from a bar magnifier.
vertically
• Zero curvature in the other
principal meridian
 Zero magnification horizontally
• Therefore more print within the field of
view
#11 Recognize patients who may benefit from a bar magnifier.
General assumptions about cylindrical
magnifiers
• Object plane is at center of
curvature, C, of the cylinder
• Therefore image plane is the
same as the object plane
If we assume:
n = 1.5 = index of refraction of plastic magnifier
n’ = 1 (for air surrounding the magnifier)
Object plane is at center of curvature of cylinder:
n’ = 1.0
Thank you!
n = 1.5
C, object plane, & image plane,
but image 50% larger than object
#11 Recognize patients who may benefit from a bar magnifier.
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